1. Field of the Invention
The present invention relates to a substrate for an ink jet head having a protective element for electrically protecting internal elements. The substrate is used for an ink jet head operable to record by discharging ink droplets from discharge ports. The invention is further directed to an ink jet head having such a substrate for the ink jet head and an ink jet head recording apparatus having such an ink jet head.
2. Related Background Art
Conventionally, an ink jet recording method of recording on a recording medium (papers in most cases) via discharge and flow of ink droplets from discharge ports is known. This ink jet recording method, which is a non-impact type recording method, has recently been popularized rapidly because it has relatively low noise generation, is capable of recording directly on a paper, and also is capable of easily recording color images by using multi-colors of ink. Among a variety of recording methods, particularly known is that of forming ink bubbles by applying thermal energy to ink responsive to recording signals and then, with an action force generated thereupon, discharging and flowing the ink from discharge ports. This method has an advantage in that an ink jet head with high density multi-nozzles capable of providing high-resolution high-speed recording may be easily realized and obtained.
An ink jet head used for this recording method is, in most cases, provided with a number of discharge ports for discharging ink, liquid channels each of which is provided for the discharge port and is in communication therewith, and a common liquid chamber for stably supplying ink into each liquid channel. This ink jet head utilizes thermal energy generated when a heater is energized through a driver, and thereby discharges ink delivered from the liquid channels from the discharge ports for a recording operation.
Such an ink jet head is so constituted, for example, that a substrate for an ink jet head is joined to a top plate on which is formed liquid channels, a liquid chamber, discharge ports, and the like. The substrate for an ink jet head comprises heaters (heating elements) for generating thermal energy to discharge ink, drivers for driving these heaters, a logic circuit for controlling the drivers, a substrate temperature sensing element for sensing substrate temperatures, a pad unit for electrically connecting the ink jet head and an ink jet recording apparatus with each other, and the like. The heaters are formed at positions corresponding to respective discharge ports and so arranged that the number of heaters may be compatible with that of the discharge ports. Therefore, the drivers are formed compatibly with the number of discharge ports. Such a substrate for an ink jet head is monolithically fabricated of a silicon semiconductor substrate according to semiconductor device manufacturing techniques. Particularly in the substrate for an ink jet head, since discharge properties of ink droplets discharged from the discharge ports in the ink jet head and substrate temperatures are closely related to each other, sensing of substrate temperatures is given a relative importance.
As the substrate temperature sensing element provided on the substrate for an ink jet head, a diode sensor is used, which may be formed on a silicon substrate by semiconductor manufacturing techniques and may provide accurate temperature measurement. By using the diode sensor, temperatures of the substrate for an ink jet head during operations or the like of the ink jet recording apparatus are sensed in accordance with temperature properties of forward voltage in a semiconductor diode.
As described above, on the substrate for an ink jet head, driver circuits, logic circuits, and the like are integrated, and the ink jet head embedded therein with such a substrate for an ink jet head is expected to be exchanged by users, so that it is so designed as to be touchable by users when exchanged. Therefore, when electrostatic discharge is generated at the time users handle the ink jet head, there sometimes arises a problem such that an electric current caused by the static electricity is applied to the substrate for the ink jet head via the pad unit of the ink jet head or via wirings, and then components weak in resisting the static electricity are damaged, leading to element breakdown. Particularly, the substrate temperature sensing element such as the diode sensor for sensing substrate temperatures is susceptible due to its weakness in resistance to electrostatic breakdown.
Accordingly, it is desired to provide a substrate for an ink jet head which has improved electrostatic breakdown resistance, an ink jet head having such a substrate for an ink jet head, and an ink jet recording apparatus using such an ink jet head.